The present invention relates to magnets for medical magnetic resonance studies and more particularly to such magnets which comprise a ferromagnetic yoke as part of the magnet structure.
Medical magnetic resonance (MR) studies are typically carried out in strong magnetic fields greater than one kilogauss. In addition to a strong magnetic field, medical magnetic resonance studies typically require a magnetic field homogeneity of the order of a few parts per million. Considerable effort has been invested in improving magnets for medical MR with a goal toward achieving the field strengths required while attaining the necessary field homogeneity over a sufficiently large spatial volume in a structure that is clinically acceptable and commercially feasible.
One technique for improving magnet efficiency is to incorporate within the magnet structure a ferromagnetic yoke which not only comprises part of the structural support but which also defines a magnetic flux return path. The use of ferromagnetic return paths in medical magnetic resonance scanner magnets is disclosed in U.S. Pat. No. 4,675,609 to Danby et.al. Ferromagnetic yoke structure is likewise disclosed in U.S. Pat. No. 4,672,346 to Miyamoto et. al. In addition to improving efficiency, the incorporation of a ferromagnetic flux return path can also be used to eliminate strong leakage magnetic fields which are inherent in aircore solenoidal magnets.
It would be desirable to incorporate ferromagnetic yokes in medical MR magnets having a strong magnetic field, greater than 5 kilogauss. The stronger the magnetic field developed by the magnet, however, the more difficult it is to achieve a magnet structure which would be considered practical by the medical community for clinical use. To avoid magnetic saturation of the ferromagnetic yoke at high field strengths, the dimensions of the yoke cross sections along the flux return path become substantial. Greater yoke cross-sectional area results, of course, in an increase in magnet weight.
Additionally, larger yoke structure can result in obstructions which hinder easy access to and egress from the patient gap of the magnet where a patient is situated during magnetic resonance scanning. Any compromises to the required yoke design from the standpoint of flux return path reluctance, in order to accommodate patient access, can materially increase the magnetic leakage field and reduce the homogeneity of the magnetic field within the gap.
Accordingly, it is an object of the invention to provide a magnet having a ferromagnetic yoke with a strong field for use in medical magnetic resonance studies which has a large gap and a structure providing easy patient access.
It is another object of the invention to provide a magnet having a ferromagnetic yoke for medical magnetic resonance scanning and having a large patient gap and a large homogeneous field region within the patient gap.
It is another object of the invention to provide a magnet for medical magnetic resonance studies which has a ferromagnetic yoke with a structure that maximizes access to the patient gap of the magnet.